This invention relates to a supervised fire alarm system in accordance with the general concept set forth hereinbelow, in which a plurality of ionization smoke detectors which may have different electrical states are connected through signal lines with control and indicating means.
Ionization type fire detectors, in which a radioactive substance generates ions, are so arranged that, upon application of an electric voltage between the electrodes of the ionization chambers, a current is generated which decreases upon penetration of smoke, or fire aerosols, into the chamber. A fire alarm system of the described type is disclosed, for instance, in U.S. Pat. No. 3,964,036.
Decrease of ion current in the ionization chamber is detected by an electrical circuit which includes a threshold detector. Upon detection, an alarm circuit can be activated. In one form the ionization chamber is connected in series with a resistance element (i.e. a load resistor). The relative voltage drops across the ionization chamber and the load resistor are sensed and applied to a threshold detector, for example, a field effect transistor (FET). If the voltage drop across the ionization measurement chamber rises, due to an increase in its resistance, the threshold value of the FET is exceeded, it begins to become conductive, and provides a fire alarm signal.
Known ionization type fire detectors are connected through signal lines to control and indication equipment (CIE). The increased FET current is conducted directly to the control and indication equipment, or over a further switching element, e.g. a monoflop with delay, or the like. The control and indicating equipment provides an alarm signal.
A problem which arises with all fire alarm systems is the occurrence of false alarms. With ionization smoke detectors there is the special problem that the detectors are sensitive to fast air currents, condensation and to the formation of a layer of dust or corrosion on the radioactive source, as these pheonmena have the same effect on the ionization current as fire aerosols. Because such a change in the ionization current increases detector sensitivity, there is an increased tendency for false alarms. The occurrence of false alarms is particularly troublesome if, as a result of an alarm, an automatic extinguishing system is activated or external fire-fighting forces are called out.
Success has been achieved in countering false slits in the sampling chamber, e.g. according to DE 2,415,479. In order to avoid the malfunctioning of ionization smoke detectors by the formation of condensation, the electrodes were heated, or the heat normally lost by the electronic circuit was utilized for heating as was suggested in DE 2,537,598.
In EP 070,449 it was suggested that the measured values be evaluated after transmission to a control unit. From the individual measured values a quiescent value for each detector is given and stored in a quiescent value memory. From the detector measured value and a comparative value stored in a comparator memory, a new comparative value is given and entered in the comparator memory. After comparing the new comparative value with a maximum rating, either a display device is activated or, from the latest detector measured value and the stored quiescent value, a new quiescent value is given and entered in the quiescent value memory. In this way it is possible to compensate for a slow change to the detector and maintain stable detector sensitivity over a long period.
In DE 2,428,325 it was suggested that condensation and degradation of insulation in the sampling chamber be avoided by using a condensation-resistant chemical compound on the plate separating the sampling and reference chambers.
In Jap. Patent Application No. JP-47-93018, it was suggested that to counter false alarms cause by soiling of the radioactive source, the dimensions of the leakage paths between the middle electrode and the two other electrodes be modified to correspond to the ratio between the chamber voltages so that, with uniform soiling, no voltage shift to the middle electrode occurs.
In order to prevent condensation of the radioactive source, which would impair the operation of the ionization smoke detector, it is suggested in DE 1,101,370 that a ring-shaped protective electrode connected to a bias voltage be installed facing the conductive source support plate. The electrical field this creates should prevent the formation of condensation on the radioactive source.
DE 2,423,046 discloses an ionization smoke detector having a protective ring system to signal any reduced insulating resistance of the sampling chamber caused by condensation or dust accumulation. A change in the potential difference between the protective ring system and the connection point between sampling and reference chambers is evaluated by the control unit as a problem indicator.
A fire alarm system is described in U.S. Pat. No. 3,964,036 in which the development of the amplified signal from the ionization smoke detector is displayed and printed out. The signal curve received is compared with known curves produced by soiling or condensation to differentiate a false alarm from a genuine alarm. This form of false alarm recognition is costly and time consuming both technically and in terms of personnel.
None of the fire alarm systems described can indicate immediately and automatically whether or not a change in the ionization current in the sampling chamber signifies a false alarm or a genuine alarm caused by a fire.
Therefore, with the foregoing in mind, it is a primary object of this invention to provide a new and improved fire detector which avoids the disadvantages of known fire detectors and which can differentiate between a genuine alarm caused by fire phenomena and a false alarm caused by the covering of the radioactive source.